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><A
NAME="SEC-EVENTHANDLING"
>Event Handling</A
></H1
><P
>The GTK signals we have already discussed are for high-level actions,
such as a menu item being selected. However, sometimes it is useful to
learn about lower-level occurrences, such as the mouse being moved, or
a key being pressed. There are also GTK signals corresponding to these
low-level <I
CLASS="EMPHASIS"
>events</I
>. The handlers for these signals have an
extra parameter which is a pointer to a structure containing
information about the event. For instance, motion event handlers are
passed a pointer to a GdkEventMotion structure which looks (in part)
like:</P
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><TD
><PRE
CLASS="PROGRAMLISTING"
>struct _GdkEventMotion
{
  GdkEventType type;
  GdkWindow *window;
  guint32 time;
  gdouble x;
  gdouble y;
  ...
  guint state;
  ...
};</PRE
></TD
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><P
><TT
CLASS="LITERAL"
>type</TT
> will be set to the event type, in this case
<TT
CLASS="LITERAL"
>GDK_MOTION_NOTIFY</TT
>, window is the window in which the event
occurred. <TT
CLASS="LITERAL"
>x</TT
> and <TT
CLASS="LITERAL"
>y</TT
> give the coordinates of the event.
<TT
CLASS="LITERAL"
>state</TT
> specifies the modifier state when the event
occurred (that is, it specifies which modifier keys and mouse buttons
were pressed). It is the bitwise OR of some of the following:</P
><TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>GDK_SHIFT_MASK  
GDK_LOCK_MASK   
GDK_CONTROL_MASK
GDK_MOD1_MASK   
GDK_MOD2_MASK   
GDK_MOD3_MASK   
GDK_MOD4_MASK   
GDK_MOD5_MASK   
GDK_BUTTON1_MASK
GDK_BUTTON2_MASK
GDK_BUTTON3_MASK
GDK_BUTTON4_MASK
GDK_BUTTON5_MASK</PRE
></TD
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><P
>As for other signals, to determine what happens when an event occurs
we call <TT
CLASS="LITERAL"
>gtk_signal_connect()</TT
>. But we also need let GTK
know which events we want to be notified about. To do this, we call
the function:</P
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><PRE
CLASS="PROGRAMLISTING"
>void gtk_widget_set_events (GtkWidget *widget,
                            gint      events);</PRE
></TD
></TR
></TABLE
><P
>The second field specifies the events we are interested in. It
is the bitwise OR of constants that specify different types
of events. For future reference the event types are:</P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>GDK_EXPOSURE_MASK
GDK_POINTER_MOTION_MASK
GDK_POINTER_MOTION_HINT_MASK
GDK_BUTTON_MOTION_MASK     
GDK_BUTTON1_MOTION_MASK    
GDK_BUTTON2_MOTION_MASK    
GDK_BUTTON3_MOTION_MASK    
GDK_BUTTON_PRESS_MASK      
GDK_BUTTON_RELEASE_MASK    
GDK_KEY_PRESS_MASK         
GDK_KEY_RELEASE_MASK       
GDK_ENTER_NOTIFY_MASK      
GDK_LEAVE_NOTIFY_MASK      
GDK_FOCUS_CHANGE_MASK      
GDK_STRUCTURE_MASK         
GDK_PROPERTY_CHANGE_MASK   
GDK_PROXIMITY_IN_MASK      
GDK_PROXIMITY_OUT_MASK     </PRE
></TD
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><P
>There are a few subtle points that have to be observed when calling
<TT
CLASS="LITERAL"
>gtk_widget_set_events()</TT
>. First, it must be called before the X window
for a GTK widget is created. In practical terms, this means you
should call it immediately after creating the widget. Second, the
widget must have an associated X window. For efficiency, many widget
types do not have their own window, but draw in their parent's window.
These widgets are:</P
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><PRE
CLASS="PROGRAMLISTING"
>GtkAlignment
GtkArrow
GtkBin
GtkBox
GtkImage
GtkItem
GtkLabel
GtkPixmap
GtkScrolledWindow
GtkSeparator
GtkTable
GtkAspectFrame
GtkFrame
GtkVBox
GtkHBox
GtkVSeparator
GtkHSeparator</PRE
></TD
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></TABLE
><P
>To capture events for these widgets, you need to use an EventBox
widget. See the section on the <A
HREF="c1228.html#SEC-EVENTBOX"
>EventBox</A
> widget for details.</P
><P
>For our drawing program, we want to know when the mouse button is
pressed and when the mouse is moved, so we specify
<TT
CLASS="LITERAL"
>GDK_POINTER_MOTION_MASK</TT
> and <TT
CLASS="LITERAL"
>GDK_BUTTON_PRESS_MASK</TT
>. We also
want to know when we need to redraw our window, so we specify
<TT
CLASS="LITERAL"
>GDK_EXPOSURE_MASK</TT
>. Although we want to be notified via a
Configure event when our window size changes, we don't have to specify
the corresponding <TT
CLASS="LITERAL"
>GDK_STRUCTURE_MASK</TT
> flag, because it is
automatically specified for all windows.</P
><P
>It turns out, however, that there is a problem with just specifying
<TT
CLASS="LITERAL"
>GDK_POINTER_MOTION_MASK</TT
>. This will cause the server to add a new
motion event to the event queue every time the user moves the mouse.
Imagine that it takes us 0.1 seconds to handle a motion event, but the
X server queues a new motion event every 0.05 seconds. We will soon
get way behind the users drawing. If the user draws for 5 seconds,
it will take us another 5 seconds to catch up after they release 
the mouse button! What we would like is to only get one motion
event for each event we process. The way to do this is to 
specify <TT
CLASS="LITERAL"
>GDK_POINTER_MOTION_HINT_MASK</TT
>. </P
><P
>When we specify <TT
CLASS="LITERAL"
>GDK_POINTER_MOTION_HINT_MASK</TT
>, the server sends
us a motion event the first time the pointer moves after entering
our window, or after a button press or release event. Subsequent 
motion events will be suppressed until we explicitly ask for
the position of the pointer using the function:</P
><TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>GdkWindow*    gdk_window_get_pointer     (GdkWindow       *window,
					  gint            *x,
					  gint            *y,
					  GdkModifierType *mask);</PRE
></TD
></TR
></TABLE
><P
>(There is another function, <TT
CLASS="LITERAL"
>gtk_widget_get_pointer()</TT
> which
has a simpler interface, but turns out not to be very useful, since
it only retrieves the position of the mouse, not whether the buttons
are pressed.)</P
><P
>The code to set the events for our window then looks like:</P
><TABLE
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><PRE
CLASS="PROGRAMLISTING"
>  gtk_signal_connect (GTK_OBJECT (drawing_area), "expose_event",
		      (GtkSignalFunc) expose_event, NULL);
  gtk_signal_connect (GTK_OBJECT(drawing_area),"configure_event",
		      (GtkSignalFunc) configure_event, NULL);
  gtk_signal_connect (GTK_OBJECT (drawing_area), "motion_notify_event",
		      (GtkSignalFunc) motion_notify_event, NULL);
  gtk_signal_connect (GTK_OBJECT (drawing_area), "button_press_event",
		      (GtkSignalFunc) button_press_event, NULL);

  gtk_widget_set_events (drawing_area, GDK_EXPOSURE_MASK
			 | GDK_LEAVE_NOTIFY_MASK
			 | GDK_BUTTON_PRESS_MASK
			 | GDK_POINTER_MOTION_MASK
			 | GDK_POINTER_MOTION_HINT_MASK);</PRE
></TD
></TR
></TABLE
><P
>We'll save the "expose_event" and "configure_event" handlers for
later. The "motion_notify_event" and "button_press_event" handlers
are pretty simple:</P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>static gboolean
button_press_event( GtkWidget *widget, GdkEventButton *event )
{
  if (event-&#62;button == 1 &#38;&#38; pixmap != NULL)
      draw_brush (widget, event-&#62;x, event-&#62;y);

  return TRUE;
}

static gboolean
motion_notify_event( GtkWidget *widget, GdkEventMotion *event )
{
  int x, y;
  GdkModifierType state;

  if (event-&#62;is_hint)
    gdk_window_get_pointer (event-&#62;window, &#38;x, &#38;y, &#38;state);
  else
    {
      x = event-&#62;x;
      y = event-&#62;y;
      state = event-&#62;state;
    }
    
  if (state &#38; GDK_BUTTON1_MASK &#38;&#38; pixmap != NULL)
    draw_brush (widget, x, y);
  
  return TRUE;
}</PRE
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